Silicon wafer and method for manufacturing the same

ABSTRACT

A silicon wafer and a method for manufacturing the same are provided, wherein the silicon wafer has no crystal defects in the vicinity of the surface and provides excellent gettering efficiency in the process of manufacturing devices without IG treatment. The oxygen concentration and the carbon concentration are controlled respectively within a range of 11×10 17 -17×10 17  atoms/cm 3  (OLD ASTM) and within a range of 1×10 16 -15×10 16  atoms/cm 3  (NEW ASTM). A denuded zone having no crystal defects due to the existence of oxygen is formed on the surface and in the vicinity thereof, and oxygen precipitates are formed at a density of 1×10 4 -5×10 6  counts/cm 2 , when a heat treatment is carried out at a temperature of 500-1000° C. for 1 to 24 hours. In the method for manufacturing the silicon wafer, moreover, the silicon wafer having the oxygen and carbon concentrations as controlled above is heat-treated at a temperature of 1100° C.-1380° C. for 1 to 10 hours. The control of the oxygen and carbon concentrations in the growth of a single crystal with CZ method allows a desired density of oxygen precipitates to be attained in the process of manufacturing devices and thereby sufficient gettering efficiency to be obtained.

FIELD OF THE INVENTION

[0001] The present invention relates to a silicon wafer used as asemiconductor substrate and a method for manufacturing such a siliconwafer, and more specifically to a silicon wafer and a method formanufacturing such a silicon wafer, wherein the number of crystaldefects on the surface or in the vicinity of the surface of the wafer,which negatively influence the device properties, can be reduced, andoxygen precipitates necessary for gettering heavy metals, whichdeteriorate the device property in the process of manufacturing devices,can be generated in the wafer (hereinafter, the oxygen precipitate isoccasionally referred to as “BMD: bulk micro defect”).

DESCRIPTION OF THE PRIOR ART

[0002] In the process of manufacturing semiconductor devices, heavymetals such as Fe, Ni, Cu and the like are apt to contaminate such adevice as D-RAM at high temperature treatments. Such heavy metalcontamination causes crystal defects to be formed on the surface of asilicon wafer or in the vicinity thereof, thereby enabling variousdevice properties to be deteriorated and further the yield of productsto be reduced. Accordingly, such heavy metals as contamination sourceshave to be removed from the active areas of the device on the wafersurface and in the vicinity thereof.

[0003] In view of this fact, a treatment is traditionally carried out toform oxygen precipitates for gettering heavy metals, wherein a waferbefore the process of manufacturing devices is heat-treated at a lowtemperature so as to grow the oxygen precipitating nuclei in the insidethereof (hereinafter, this treatment is referred to as “IG treatment”,i.e., Intrinsic Gettering treatment).

[0004] Moreover, a further enhancement of the quality of a wafer on thesurface and in the vicinity thereof is required in manufacturing ahigh-integrated device. For this purpose, a silicon wafer isheat-treated at a high temperature more than 1000° C. to diffuse or moveoxygen atoms on the surface of the wafer and in the vicinity thereof tothe outside of the wafer, so that a defect-free layer is formed in anarea (a denuded zone) where the crystal defects due to the existence ofoxygen are removed.

[0005] In the conventional process of manufacturing devices at a hightemperature, where a heat treatment for buried layers is employed, asilicon wafer is heat-treated at a high temperature of 1120° C.-1220° C.As a result, a defect-free layer or a denuded zone can be formed on thesurface of the wafer and in the vicinity thereof, so that it isunnecessary to form such a denuded zone before the process ofmanufacturing devices. However, a high-energy ion implantation processis conventionally employed in the recent process of manufacturingdevices so that the heat treatment is carried out at a relatively lowtemperature. This procedure makes it difficult to form the denuded zone.As a result, it is necessary to form such a denuded zone in the waferbefore the process of manufacturing devices.

[0006] Silicon wafers are normally prepared from a single crystalproduced with the Czochralski method (hereinafter, referred to as “CZmethod”), so that a number of oxygen precipitate nuclei are formed inthe inside of the wafer during the crystal growth. In this case, a lowtemperature heat treatment causes the growth of the oxygen precipitatenuclei to be grown and thus oxygen precipitates having a larger size tobe provided, thereby making it possible to enhance the function ofgettering heavy metals as the sources for polluting the wafer.

[0007] When a silicon wafer is subjected to a high temperature heattreatment for forming the denuded zone (hereinafter, this treatment isreferred to as “DZ treatment”), the oxygen precipitate nuclei in theinside of the silicon wafer shrink and finally disappear. Therefore, theDZ treatment causes the density of the oxygen precipitates to bereduced, so that the gettering efficiency is reduced in the process ofmanufacturing the devices.

[0008] In order to solve such a problem, a two-step heat treatment isconventionally employed, that is, the DZ treatment is firstly applied toa silicon wafer before the process of manufacturing devices, and thenthe so called IG treatment for the growth of the oxygen precipitatingnuclei is applied to the silicon wafer (hereinafter, this treatment isreferred to as “DZ-IG treatment”). More specifically, the DZ treatmentis applied to the wafer in the first step, and therefore oxygen atoms onthe surface and in the vicinity thereof are moved to the outside of thewafer so as to eliminate the crystal defects resulting from theexistence of the oxygen atoms, thereby enabling the denuded zone to beformed. In the second step, the IG treatment is applied to the wafer togrow the oxygen precipitate nuclei in the inside thereof, and thus theoxygen precipitates as gettering sources are generated.

[0009] For instance, in Japanese Patent Application Laid-openPublication No. 61-15335, a DZ-IG treatment is described, wherein awafer, which is prepared from a silicon single crystal grown with the CZmethod, is subjected to a diffusion treatment for moving oxygen atomsfrom the surface of the wafer to the outside at a temperature not lessthan 1100° C. Thereafter, the wafer is once cooled and then heated at arate of 1-5° C./min in a temperature range from not more than 600° C. tonot less than 800° C. and finally the wafer is subjected to a heattreatment at 1000-1200° C.

[0010] In such a DZ-IG treatment, however, the oxygen precipitatingnuclei in the inside of the silicon wafer shrink and then disappear,because the temperature of the DZ treatment in the first step is stillhigh for the silicon wafer. As a result, even if the IG treatment in thesecond step is applied, the density of the oxygen precipitates in theinside of the wafer is still small, so that, in the IG treatment, a longtime is required to grow the oxygen precipitates at a high density.

[0011] On the other hand, device users require a higher quality of thesilicon wafer and, at the same time, a further reduction in the costthereof. Therefore, it is necessary to provide an improved method formanufacturing a high quality silicon wafer at a reduced cost. In view ofthese facts, the above-mentioned procedure, i.e., the IG treatment for along time after the DZ treatment can no longer be employed, because itcauses both the number of process steps and the production cost to beincreased, and therefore it does not meet the above-mentioned user'srequirements.

[0012] An epitaxial wafer includes neither defects resulting from theexistence of oxygen nor grown-in defects (containing COP: crystaloriginated particles) formed in growing a silicon single crystal in theepitaxial layer of the surface, on which devices are fabricated.Accordingly, the epitaxial wafer can be used as a silicon waferincluding B, As/Sb and the like at a greater concentration, so that thewafer can be used for manufacturing a high performance device such asMPU, flash memory or the like as well as a high performance power devicesuch as MOSFET, IGBT or the like.

[0013] When, however, either defects resulting from the existence ofoxygen or grown-in defects (containing COP) are already included in thesurface of a silicon wafer which is used as an epitaxial wafer, thesedefects serve as nuclei for forming secondary defects in the epitaxiallayer, and therefore this causes the characteristic of the device to bedeteriorated.

[0014] In the recent process of manufacturing highly integrated devicesat a high density, the heat treatment is carried out at a relatively lowtemperature, and the IG treatment is applied to such an epitaxial waferin order to avoid both the contamination of the wafer and the formationof crystal defects in the device process. However, the epitaxial growthis carried out at a higher temperature and therefore the oxygenprecipitating nuclei in the inside of the wafer decrease and eventuallydisappear. As a result, the number of the oxygen precipitates to beformed in the IG treatment is decreased, thereby causing the getteringefficiency to be reduced in the process of manufacturing the devices.

SUMMARY OF THE INVENTION

[0015] In view of the above-mentioned problems, it is an object of thepresent invention to provide a silicon wafer which ensures a highefficiency for gettering heavy metals as contamination sources in theprocess of manufacturing the devices, and which is capable of forming adenuded zone on the surface of the wafer and in the vicinity thereofwithout applying any IG treatment to the silicon wafer.

[0016] It is another object of the present invention to provide a methodfor manufacturing such a silicon wafer.

[0017] The present inventors intensively carried out an experimentalinvestigation for the purpose of producing a silicon wafer capable ofproviding sufficient gettering efficiency, even if any IG treatment isfurther not carried out in the process of manufacturing devices. Theresults obtained in the experimental works reveal that the density ofoxygen precipitating nuclei in a silicon single crystal increases withthe increase of the oxygen concentration and that the density of oxygenprecipitates also increases with the increase of the carbonconcentration, although detailed mechanism cannot be ascertained.

[0018] Furthermore, it is found that an increased carbon concentrationin the silicon wafer does not provide a desired amount of oxygenprecipitates even in the application of the low temperature heattreatment, unless a predetermined level of the oxygen concentration issatisfied.

[0019] So long as oxygen and carbon atoms are intentionally injectedinto the silicon wafer and the concentration of each of the two typeatoms is adjusted within a corresponding predetermined level, the oxygenprecipitating nuclei grow by applying the low temperature heat treatmentin the process of manufacturing devices to generate a sufficient amountof oxygen precipitates, even if the high temperature heat treatment isapplied beforehand to the silicon wafer in the DZ treatment. As aresult, it is possible to getter heavy metals as source of contaminatingthe wafer.

[0020] In accordance with the present invention, the following subjectmatters, (1) a silicon wafer and (2) a method for manufacturing thesilicon wafer, can be realized, based on the above knowledge in theexperimental investigation:

[0021] (1) A silicon wafer in which the oxygen concentration and thecarbon concentration are controlled respectively within a range of11×10¹⁷-17×10¹⁷ atoms/cm³ (OLD ASTM) and within a range of1×10¹⁶-15×10¹⁶ atoms/cm³ (NEW ASTM), wherein a denuded zone having nocrystal defects due to the existence of oxygen is formed on the surfaceand in the vicinity thereof, and wherein oxygen precipitates are formedat a density of 1×10⁴ counts/cm²-5×10⁶ counts/cm², when a heat treatmentis carried out at 500-1000° C. for 1 to 24 hours.

[0022] (2) A method for manufacturing a silicon wafer, said methodcomprising the following steps of: controlling the oxygen concentrationand the carbon concentration respectively within a range of11×10¹⁷-17×10¹⁷ atoms/cm³ (OLD ASTM) and within a range of1×10¹⁶-15×10¹⁶ atoms/cm³ (NEW ASTM), and heat-treating said siliconwafer at 1100° C.-1380° C. for 1-10 hours.

[0023] In conjunction with the above, it is preferable that thethickness of a denuded zone formed on the surface or in the vicinity ofthe surface of the silicon wafer is not less than 10 μm. Moreover, it ispreferable that the heat treatment in the above-mentioned method formanufacturing the silicon wafer is carried out at a temperature of 1100°C.-1380° C. under an atmosphere of inert gas, or a mixture of inert gasand oxidizable gas, or hydrogen gas, or hydrogen-containing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a drawing showing temperature patterns in two typeevaluation heat treatments in an assumed process of manufacturingdevices;

[0025]FIG. 2 is a diagram showing the relationship between the densityof oxygen precipitates in silicon wafers intentionally contaminated withNi and the yield of devices having gate oxide integrity;

[0026]FIG. 3 is a diagram showing the density of oxygen precipitatesafter the evaluation heat treatment in accordance with the sample levelsof Example 1;

[0027]FIG. 4 is a diagram showing the thickness of a denuded zone afterthe evaluation heat treatment in accordance with the sample levels ofExample 1;

[0028]FIG. 5 is a diagram showing the relationship between the carbonconcentration and the length of slip dislocations generated in a wafer;

[0029]FIG. 6 is a diagram showing the density of oxygen precipitatesafter the evaluation heat treatment in accordance with the sample levelsof Example 2; and

[0030]FIG. 7 is a diagram showing the thickness of a denuded zone afterthe evaluation heat treatment in accordance with the sample levels ofExample 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] In a silicon wafer according to the present invention, no IGtreatment is employed before the process of manufacturing devices.However, oxygen precipitates are generated in the silicon wafer at aconcentration of 1×10⁴ counts/cm²-5×10⁶ counts/cm² by a low temperatureheat treatment in the process of manufacturing devices by controllingeach of the oxygen concentration and the carbon concentration within apredetermined level in the silicon wafer, although a denuded zone isformed on the surface of the wafer by applying a high temperature heattreatment in the DZ treatment.

[0032] In accordance with the present invention, as described above, asilicon wafer can be produced at a low cost, since no IG treatment iscarried out after applying such a high temperature treatment in the DZtreatment to the wafer.

[0033] In the silicon wafer according to the present invention, oxygenprecipitates are generated at a density of 1×10⁴ counts/cm²-5×10⁶counts/cm², when a heat treatment is carried out for 1 hour-24 hours ata temperature of 500° C.-1000° C. Such an amount of oxygen precipitatesis sufficient to obtain an excellent gettering effect in the process ofmanufacturing devices with a low temperature heat treatment.

[0034] However, the present invention is not always restricted to theabove-mentioned silicon wafer, which is used in the process ofmanufacturing devices with the low temperature heat treatment. When asilicon wafer according to the invention is used in the process ofmanufacturing devices with both the low temperature heat treatment andthe high temperature heat treatment, the oxygen precipitates are formedinside the silicon wafer at a density of more than 1×10⁴ counts/cm²,thereby enabling sufficient getterring efficiency to be obtained.

[0035] In the silicon wafer according to the present invention,sufficient gettering efficiency can be obtained at a density of oxygenprecipitates more than 1×10⁴ counts/cm², so that the deterioration ofdevice properties, such as the gate oxide integrity and others, can beavoided. However, an excessive density of oxygen precipitates causes themechanical strength due to the excessive precipitation to be reduced. Asa result, it is preferable that the upper limit of the density should beset at 5×10⁶ counts/cm².

[0036] In the method for manufacturing a silicon wafer according to thepresent invention, the silicon wafer is heat-treated at a hightemperature of 1100° C.-1380° C. to form a denuded zone by reducing oreliminating the crystal defects. In particular, the heat treatment at ahigh temperature of 1280° C.-1380° C. is effective to reduce oreliminate the grown-in defects in the vicinity of the wafer surface.

[0037] In this case, it is necessary to carry out the heat treatment for1 to 10 hours. A heating time of less than one hour provides aninsufficient formation of the denuded zone on the wafer surface, whereasa heating time of more than 10 hours causes the wafer to be softened andeventually deformed due to a very high temperature in the heattreatment.

[0038] In the method for manufacturing a silicon wafer according to thepresent invention, it is preferable that the atmosphere in the hightemperature treatment comprises inert gas (argon gas, nitrogen gas, orthe like) or a mixture of inert gas and oxidizable gas (oxygen gas) inorder to protect the surface of the wafer. However, a high temperatureheat treatment under inert gas atmosphere causes the surface state ofthe wafer to be deteriorated and, therefore, it is particularlypreferable to carry out the heat treatment under a mixed gas atmospherecontaining a small amount of oxidizable gas in an inert gas.Furthermore, in order to eliminate the grown-in defects in the wafer, itis preferable that the heat treatment is carried out under an atmosphereconsisting of hydrogen gas or hydrogen-containing gas.

EXAMPLES

[0039] The advantages of the silicon wafer according to the presentinvention will be described in several examples for the process ofmanufacturing devices. For this purpose, assuming an actual treatmentused in the process of manufacturing devices, the evaluation heattreatment was applied to a silicon wafer and then the properties thereofwere studied.

[0040]FIG. 1 shows two diagrams including two type programs of heatapplication in the evaluation heat treatment for the process ofmanufacturing devices. As shown in FIG. 1, the evaluation heat treatmentcan be classified into a high temperature device manufacturing process[α]including heat treatments at a temperature of 1100° C. or greater(hereinafter, referred to as “X high temperature process”), and a lowtemperature device manufacturing process [Y] including heat treatmentsat a temperature less than 1100° C. (hereinafter, referred to as “Y lowtemperature process”).

Example 1

[0041] A p-type <100> silicon wafer having a specific resistivity of 10Ω·cm was prepared, using the CZ method, in which case, the silicon waferincluded oxygen at a fixed concentration of 14×10¹⁷ atoms/cm³ and carbonat a varied concentration of 1×10¹⁶ atoms/cm³-16×10¹⁶ atoms/cm³.

[0042] In the high temperature heat treatment, the DZ treatment wascarried out at a temperature of 1100° C.-1350° C. for one hour undernitrogen gas atmosphere including oxygen gas in a 3% concentration.However, no high temperature heat treatment was carried out in part ofComparative Examples (No. 21-24). Conditions for sample level of theprepared silicon wafers are listed in Table 1. TABLE 1 High EvaluationOxygen Carbon Temperature Sample Heat Concentration Concentration HeatTreatment Classification No. Level Treatment (atoms/cm³) (atoms/cm³)Condition Inventive 1 A X process 14 × 10¹⁷   1 × 10¹⁶ 1100° C. × 1 hrExample 2 A Y process 14 × 10¹⁷   1 × 10¹⁶ 1100° C. × 1 hr 3 B X process14 × 10¹⁷   5 × 10¹⁶ 1100° C. × 1 hr 4 B Y process 14 × 10¹⁷   5 × 10¹⁶1100° C. × 1 hr 5 C X process 14 × 10¹⁷  10 × 10¹⁶ 1100° C. × 1 hr 6 C Yprocess 14 × 10¹⁷  10 × 10¹⁶ 1100° C. × 1 hr 7 D X process 14 × 10¹⁷  15× 10¹⁶ 1100° C. × 1 hr 8 D Y process 14 × 10¹⁷  15 × 10¹⁶ 1100° C. × 1hr 9 E X process 14 × 10¹⁷   1 × 10¹⁶ 1350° C. × 1 hr 10 E Y process 14× 10¹⁷   1 × 10¹⁶ 1350° C. × 1 hr 11 F X process 14 × 10¹⁷   5 × 10¹⁶1350° C. × 1 hr 12 F Y process 14 × 10¹⁷   5 × 10¹⁶ 1350° C. × 1 hr 13 GX process 14 × 10¹⁷  10 × 10¹⁶ 1350° C. × 1 hr 14 G Y process 14 × 10¹⁷ 10 × 10¹⁶ 1350° C. × 1 hr 15 H X process 14 × 10¹⁷  15 × 10¹⁶ 1350° C.× 1 hr 16 H Y process 14 × 10¹⁷  15 × 10¹⁶ 1350° C. × 1 hr Comparative17 I X process 14 × 10¹⁷ None 1100° C. × 1 hr Example 18 I Y process 14× 10¹⁷ None 1100° C. × 1 hr 19 J X process 14 × 10¹⁷ None 1350° C. × 1hr 20 J Y process 14 × 10¹⁷ None 1350° C. × 1 hr 21 K X process 14 ×10¹⁷   1 × 10¹⁶ None 22 K Y process 14 × 10¹⁷   1 × 10¹⁶ None 23 L Xprocess 14 × 10¹⁷  15 × 10¹⁶ None 24 L Y process 14 × 10¹⁷  15 × 10¹⁶None 25 M X process 14 × 10¹⁷ 0.5 × 10¹⁶ 1100° C. × 1 hr 26 M Y process14 × 10¹⁷ 0.5 × 10¹⁶ 1100° C. × 1 hr 27 N X process 14 × 10¹⁷  16 × 10¹⁶1350° C. × 1 hr 28 N Y process 14 × 10¹⁷  16 × 10¹⁶ 1350° C. × 1 hr

[0043] Firstly, the density of oxygen precipitates in the silicon waferwas measured in order to ascertain the efficiency for gettering heavymetals as contamination sources in the wafer. For this purpose, therelationship between the density of oxygen precipitates and the yield ofdevices having gate oxide integrity in a silicon wafer intentionallycontaminated with Ni was investigated.

[0044] A wafer having sample level E was used as for a silicon waferintentionally contaminated with Ni, and an additional heat treatment forprecipitating oxygen at 700° C. for 1-8 hours was applied to the waferto vary the density of oxygen precipitates. In this case, the X hightemperature process was employed for the evaluation heat treatmentemployed. After the evaluation heat treatment, the wafer wasintentionally contaminated with Ni to obtain a Ni concentration of1×10¹¹ atoms/cm². Furthermore, after a heat treatment for diffusing suchNi contamination into the wafer was applied, electrodes having a MOSstructure were fabricated on the surface of the wafer, and then theyield of devices having gate oxide integrity was determined.

[0045]FIG. 2 is a diagram showing the relationship between the densityof oxygen precipitates and the yield of devices having gate oxideintegrity in a silicon wafer intentionally contaminated with Ni. In thediagram, it can be recognized that a decreased density of oxygenprecipitates down to not more than 1×10⁴ counts/cm² causes the withstandvoltage of the oxide layer to be deteriorated and the yield of deviceshaving gate oxide integrity to be greatly reduced.

[0046] Such deterioration results from the fact that the oxygenprecipitates are all gettered by a restricted amount of Ni atoms and theresidual Ni atoms form Ni silicide on the surface of the wafer. From theresult in FIG. 2, it can be recognized that, as for the waferintentionally contaminated with Ni, a reduction of the withstand voltageof the oxide layer can be avoided at a density of oxide precipitates ofnot less than 1×10⁴ counts/cm². However, an excessive density of oxygenprecipitates may provide a reduction of the mechanical strength due tothe excessive amount of precipitates. Taking this fact into account, theupper limit of the density of the oxygen precipitates is set to be 5×10⁶counts/cm².

[0047] In the following, the evaluation heat treatment was carried outfor the wafers each having one of various sample levels listed in Table1, and then the density of oxygen precipitates and the thickness of thedenuded zone were measured. More specifically, the wafer was subjectedto the X high temperature process or the Y low temperature process, andthen heat-treated at 1000° C. for 16 hours under an atmosphere ofoxidizable gas (100%) in order to observe all the oxygen precipitates.Each sample after the heat treatment was divided into two pieces, andthen a selective etching was applied thereto. Thereafter, the section ofthe wafer was observed with an optical microscope to measure both thedensity of oxygen precipitates and the thickness of the denuded zone.

[0048]FIG. 3 is a drawing showing the density of oxygen precipitatesafter the evaluation heat treatment for the wafers each having one ofthe sample levels listed in Example 1. As shown in FIG. 2, sufficientgettering efficiency can be obtained at a density of oxygen precipitatesof not less than 1×10⁴ counts/cm² for the wafers intentionallycontaminated with Ni. Accordingly, oxygen precipitates are formed at adensity of more than 1×10⁵ counts/cm² in the inventive Examples A-H (No.1-16), either with the X high temperature process or with the Y lowtemperature process, and therefore, sufficient gettering efficiency maybe expected.

[0049] On the contrary, the wafer is doped with no carbon or doped withcarbon at a very small concentration in Comparative Examples I, J (No.17 No. 20) and M (No. 25 and No. 26), so that the density of oxygenprecipitates becomes not more than 1×10⁴ counts/cm², and therefore nosufficient gettering efficiency may be expected. In Comparative ExamplesK, L (No. 21-No. 24) and N (No. 27 and No. 28), the DZ treatment is notcarried out, or the carbon concentration in the wafer is too high. As aresult, oxygen precipitates are formed at a high density of more than5×10⁶ counts/cm², and therefore there is a possibility that themechanical strength of the wafer is reduced due to an excessive amountof precipitates.

[0050]FIG. 4 is a diagram showing the thickness of the denuded zoneafter applying the evaluation heat treatment as for the sample level ofExample 1. The thickness of the denuded zone (hereinafter referred to as“DZ thickness”), which is used as an active area of a device in thevicinity of the surface of a wafer, should be adjusted so as to be adepth of 10 μm or so from the surface, although it depends on thestructure of the device to be fabricated. It is important to completelyeliminate crystal defects in such an active area of the device.

[0051] From FIG. 4, it is clear that the Inventive Examples A-H providea DZ thickness of not less than 20 μm either with the X high temperatureprocess or with the Y low temperature process. On the contrary, a DZthickness of not less than 50 μm can be obtained in Comparative ExamplesI, J and M. However, as shown in FIG. 3, no desired density of oxygenprecipitates can be obtained due to no doped carbon or due to lack ofthe carbon concentration, thereby enabling no gettering efficiency to beexpected. In Comparative Examples K and L, no DZ treatment is appliedand therefore the thickness of the denuded zone is not less than 10 μm,hence causing the device properties to be negatively influenced.

[0052] As a result, it can be stated that an adoption of the parametersregarding the carbon and oxygen concentrations and the heat treatmentspecified by the present invention provides a silicon wafer having agood quality, wherein the density of oxygen precipitates can beappropriately controlled so as to provide neither a reduction in thegettering efficiency due to a lack of oxygen nor a reduction in themechanical strength of the wafer due to an excessive amount ofprecipitation, thereby enabling the crystal defects to be reduced or tobe eliminated in the active area of the device.

[0053]FIG. 5 is a drawing showing the relationship between the carbonconcentration and the length of slip dislocations generated in a wafer.In order to obtain a desirable carbon concentration, wafers having avaried carbon concentration were prepared and an indentation test wascarried out for each wafer by compulsively applying a stress thereto.Thereafter, the wafer was heat-treated at 1000° C. for 30 min tocompulsively generate slip dislocations, and then the length of eachslip dislocation generated in the wafer was determined under theobservation using an optical microscope.

[0054] From the results of observation with the optical microscope, itcan be recognized that a decreased length of the slip dislocationsenhances the mechanical strength. All the wafers tested had an oxygenconcentration of 13.1×10¹⁷ atoms/cm³ (OLD ASTM).

[0055] From the results in FIG. 5, it is found that a sufficientmechanical strength can be obtained at a carbon concentration of 5×10¹⁶atoms/cm³ (NEW ASTM). However, doping an excessive amount of carboncauses an excessive amount of oxygen precipitates to be generated in thewafer, so that the mechanical strength of the wafer is reduced.Accordingly, it is preferable that the carbon concentration ranges from5×1016-15×10¹⁶ atoms/cm³ (NEW ASTM) to maintain sufficient mechanicalstrength for the wafer.

EXAMPLE 2

[0056] A p-type <100> silicon wafer having a specific resistivity of 10Ω·cm was prepared, using the CZ method, in which case, the silicon waferincluded carbon at a fixed concentration of 2×10¹⁶ atoms/cm³ and oxygenat a varied concentration of 11×10¹⁷ atoms/cm³-18×10¹⁷ atoms/cm³. A hightemperature heat treatment was carried out both at 1000° C. for one hourand at 1200° C. for one hour under a nitrogen gas atmosphere containingoxygen gas at a concentration of 3%. The sample level of the preparedwafers is shown in Table 2. TABLE 2 High Evaluation Oxygen CarbonTemperature Sample Heat Concentration Concentration Heat TreatmentClassification No. Level Treatment (atoms/cm³) (atoms/cm³) ConditionInventive 29 O Y process 11 × 10¹⁷ 2 × 10¹⁶ 1200° C. × 1 hr Example 30 PY process 17 × 10¹⁷ 2 × 10¹⁶ 1200° C. × 1 hr 31 Q Y process 11 × 10¹⁷ 5× 10¹⁶ 1200° C. × 1 hr 32 R Y process 17 × 10¹⁷ 5 × 10¹⁶ 1200° C. × 1 hrComparative 33 S Y process 10 × 10¹⁷ 2 × 10¹⁶ 1200° C. × 1 hr Example 34T Y process 18 × 10¹⁷ 2 × 10¹⁶ 1200° C. × 1 hr 35 U Y process 11 × 10¹⁷2 × 10¹⁶ 1000° C. × 1 hr 36 V Y process 17 × 10¹⁷ 2 × 10¹⁶ 1000° C. × 1hr

[0057] In Example 2, each sample of wafer was applied to an evaluationheat Treatment of the Y low temperature process, and then heat-treatedat 1000° C. for 16 hours under an atmosphere of oxidizable gas (100%) toobserve all the oxygen precipitates in the wafer. After a lowtemperature heat treatment, each sample was divided into two pieces andselectively etched Thereafter, the section of each sample was observedwith an optical microscope to determine both the density of oxygenprecipitates and the thickness of the denuded zone.

[0058]FIG. 6 is a diagram showing the density of oxygen precipitatesafter the evaluation heat treatment in the sample levels of Example 2.FIG. 7 is a diagram showing the DZ thickness after applying theevaluation heat treatment in the sample levels of Example 2. From bothdiagrams, it is found that in the Inventive Example O-R (No. 29-32),oxygen precipitates at a density of not less than 1×10⁴ counts/cm² areformed, thereby enabling a sufficient gettering efficiency to beobtained, and further a DZ thickness of not less that 20 μm or at leastnot less than 10 μm can be obtained.

[0059] In Comprative Example S (No. 33), however, the density of oxygenprecipitates is not more than 1×10⁴ counts/cm², thereby enabling nosufficient gettering efficiency to be expected. In Comparative ExamplesT, U, V (No. 34-36), a sufficient density of oxygen precipitates can beobtained, but the DZ thickness is less than 10 μm. Such a reduced amountof the thickness causes the device properties to be deteriorated.

[0060] In the above Examples 1 and 2, all the DZ treatments were carriedout under a mixed gas atmosphere of oxygen and nitrogen gases. When themixed gas was replaced with hydrogen gas, substantially the same effectwas obtained as for the density of oxygen precipitates, and it wasconfirmed that the grown-in defects were more prominently eliminated inthe denuded zone.

[0061] As described above, in the silicon wafer and the method formanufacturing the same according to the present invention, both theoxygen concentration and the carbon concentration are controlled in theprocess of growing a single crystal with the CZ method, so that adesired amount of the density of precipitates can be obtained in theprocess of fabricating the device without application of the IGtreatment after the DZ treatment, thereby enabling sufficient getteringefficiency to be obtained. Moreover, no requirement of the IG processcauses the productivity to be enhanced with a reduced cost. When thewafer is used as an epitaxial wafer, no defects are generated in anepitaxial layer, since a denuded zone has already been formed on thesurface of the wafer before the epitaxial layer is grown. As a result,desired gettering efficiency can similarly be obtained in the process ofmanufacturing devices.

What is claimed is:
 1. A silicon wafer in which the oxygen concentrationand the carbon concentration are controlled respectively within a rangeof 11×10¹⁷-17×10¹⁷ atoms/cm³ (OLD ASTM) and within a range of1×10¹⁶-15×10¹⁶ atoms/cm³ (NEW ASTM), wherein a denuded zone having nocrystal defects due to the existence of oxygen is formed on the surfaceand in the vicinity thereof, and wherein oxygen precipitates are formedat a density of 1×10⁴ counts/cm²-5×10⁶ counts/cm², when a heat treatmentis carried out at 500-1000° C. for 1 to 24 hours.
 2. A silicon waferaccording to claim 1, wherein the thickness of said denuded zone is notless than 10 μm.
 3. A silicon wafer according to claim 1, wherein saidcarbon concentration is controlled within a range of 5×10¹⁶-15×10¹⁶atoms/cm³ (NEW ASTM).
 4. A silicon wafer according to claim 3, whereinthe thickness of said denuded zone is not less than 10 μm.
 5. A methodfor manufacturing a silicon wafer, said method comprising the followingsteps of: controlling the oxygen concentration and the carbonconcentration respectively within a range of 11×10¹⁷-17×10¹⁷ atoms/cm³(OLD ASTM) and within a range of 1×10¹⁶-15×10¹⁶ atoms/cm³ (NEW ASTM),and heat-treating said silicon wafer at 1100° C.-1380° C. for 1 hour-10hours.
 6. A method for manufacturing a silicon wafer according to claim5, wherein said heat treatment at 1100° C.-1380° C. is carried out underan atmosphere of inert gas or a mixture of inert gas and oxidizable gas.7. A method for manufacturing a silicon wafer according to claim 5,wherein said heat treatment at 1100° C.-1380° C. is carried out under anatmosphere of hydrogen gas or gas containing hydrogen.
 8. A method formanufacturing a silicon wafer according to claim 5, wherein thethickness of a denuded zone formed on the surface and in the vicinitythereof is not less than 10 μm.
 9. A method for manufacturing a siliconwafer according to claim 5, wherein said carbon concentration iscontrolled within a range of 5×10¹⁶-15×10¹⁶ atoms/cm³ (NEW ASTM).
 10. Amethod for manufacturing a silicon wafer according to claim 9, whereinsaid heat treatment at 1100° C.-1380° C. is carried out under anatmosphere of inert gas or a mixture of inert gas and oxidizable gas.11. A method for manufacturing a silicon wafer according to claim 9,wherein said heat treatment at 1100° C.-1380° C. is carried out under anatmosphere of hydrogen gas or gas containing hydrogen.
 12. A method formanufacturing a silicon wafer according to claim 9, wherein thethickness of a denuded zone formed on the surface and in the vicinitythereof is not less than 10 μm.